There is known an active silencer for a sound transmitted in a one-dimensional manner such as a sound radiated from an exhaust duct of a generator engine. The active silencer is called a one-dimension active silencer, and used in many fields.
FIG. 21 is a schematic view of a conventional one-dimension active silencer. As shown in FIG. 21, in opposition to a sound transmitted from a sound source 52 arranged at an end of the exhaust duct 51, the conventional one-dimension active silencer 50 is intended to reduce a sound power at an outlet 53, which is the other end of the exhaust duct 51, to zero.
The one-dimension active silencer 50 has a microphone 54 arranged at a side of the sound source 52 in the exhaust duct 51 in order to detect a sound signal in the exhaust duct 51. The one-dimension active silencer 50 has also a similar microphone 55 arranged at a side of the outlet 53 in the exhaust duct 51 in order to detect a sound signal in the exhaust duct 51.
A speaker 56 which can generate an additional sound is provided between the two microphones 54 and 55 in the exhaust duct 51. An adaptation controller 57 is provided for controlling the speaker 56. The adaptation controller 57 is adapted to control the speaker 56 based on a Filtered-X LMS algorithm, which is often used in general adaptation controlling systems.
The adaptation controller 57 has a compensation filter 58 for a transmitting function, a fixed FIR (Finite Impulse Response) filter 59 and an adaptive FIR filter 60. An output signal from the microphone 54 is adapted to be inputted to the compensation filter 58 and the fixed FIR filter 59. An output signal from the microphone 55 is adapted to be inputted to the adaptive FIR filter 60. An output from the fixed FIR filter 59 is adapted to be inputted to the speaker 56. Then, the adaptation controller 57 is adapted to automatically determine controlling coefficients in such a manner that the sound level detected by the microphone 55 is substantially zero.
In such a one-dimension active silencer 50, based on the sound detected by the microphone 54, the speaker 56 generates an additional sound having a reverse phase with respect to the detected sound. Thus, the additional sound interferes with the sound transmitted from the sound source 52 in the exhaust duct 51. The result of the interference is monitored by the microphone 55. Then, the speaker 56 continues to be controlled to generate an additional sound in such a manner that the sound power is reduced to substantially zero at the position where the microphone 55 is arranged. Once the sound power is reduced to zero at a position, the position reflects the sound because of difference in sound impedance. Therefore, the sound is not transmitted to the outlet 53 any more.
As described above, the one-dimension active silencer 50 cause the sound power from the sound source to interfere with the additional sound and to reduce to substantially zero when the sound power is transmitted in a one-dimensional manner, for example when the sound power is transmitted through the exhaust duct 51. However, the one-dimension active silencer 50 is not effective in reducing a sound power transmitted in a three-dimensional manner.
There is known a method for reducing a total acoustic power when a sound power from a sound source such as a machine is radiated to a three-dimension space. In the method, many additional sound sources are arranged around the sound source. The respective additional sound sources generate respective additional sounds in order to reduce a sound power leaked from a space surrounded by the additional sound sources as much as possible. Such a conventional three-dimension active silencer is schematically shown in FIG. 22.
As shown in FIG. 22, a plurality of additional sound sources 113 are arranged on a surrounding spherical surface 112 around a machine 111 as a target sound source. A plurality of microphone 114 is arranged close to the plurality of additional sound sources 113, respectively.
Each of the plurality of microphones 114 is connected to a common controller 115 which is adapted to drive and control the plurality of additional sound sources 113. A reference signal relating to a sound radiated from the machine 111, for example a vibration signal of an engine if the machine 111 is the engine, is adapted to be inputted into the controller 115.
The controller 115 conducts a control based on the reference signal in such a manner that each of the sound levels detected by the microphones 114 is reduced to a substantially minimum, respectively. That is, the controller 115 determines an amplitude and a phase of each of the additional sound sources 113 in such a manner that a sum of squares of the sound levels detected by the microphones 114 is a minimum.
However, in the silencer system shown in FIG. 22, each of the microphones 114 detects a plurality of additional sounds radiated from the plurality of additional sound sources 113, at the same time (multi-overlapping). Thus, a controlling system in the controller 115 has to be built considering an affect of the multi-overlapping, which makes the controlling system more complex. In addition, a gap between each neighboring two of the additional sound sources 113 has to be less than half a wavelength of the sound from the target sound source. Thus, if the spherical surface 112 is defined far from the machine (target sound source) 111, the number of the additional sound sources 113 has to be increased.
In addition, although the conventional active silencing control can achieve lowest sound powers at the positions where the microphones 114 are placed, it does not necessarily mean that a total acoustic power is a substantially minimum. That is, it is still not achieved to control to reduce a total acoustic power in a three-dimension space including a machine as a sound source.
On the other hand, there are known some passive silencing controls such as sound absorption or sound shading. However, the passive silencing controls is not so effective, especially when a main component of the sound is a low tone for example when the sound is radiated from a generator.